Free radically curable oligomers and polymers are well-known in the art, finding utility, for example, in the graphic arts, in the adhesives and coatings industry, and in a variety of biomedical areas. For the most part, these oligomers and polymers contain ethylenically unsaturated functional groups (often acrylate or methacrylate esters) at the termini of the polymer or pendant to the polymer chain. In general, these curable polymers are prepared from oligomers or polymers having various reactive functional groups and ethylenically unsaturated molecules having complementary reactive functional groups. Due to the wide variety of hydroxy functional oligomers and polymers which are available commercially, it is desirable to have efficient methods whereby these polymers can be converted into free radically curable polymers.
A method for the preparation of free radically curable oligomers which has advantages over prior art methods is taught in assignee's copending patent application U.S. Ser. No. 316,234 filed Oct. 29, 1981, and published European Patent Application No. 0 091 956, wherein acrylamide and methacrylamide functional oligomers are prepared by reaction of a nucleophilic group-substituted oligomer with an alkenyl azlactone. With hydroxy functional oligomers, this application exemplifies the use of certain Lewis acids e.g., aluminum chloride) as efficient catalysts for reaction with vinyl azlactones (e.g., 4,4 dimethyl-2-ethenyl-2-oxazolin-5-one). Although this process does allow the preparation of acrylamide functional oligomers, side reactions (leading to chain extension and in some cases crosslinking) can result in lower acrylamide functionality (i.e., higher acrylamide equivalent weight) than is theoretical. Correspondingly, assignee's copending patent application U.S. Ser. No. 019,473, filed Feb. 26, 1987, describes an improved process in which a hydroxy functional oligomer or polymer is reacted with an isopropenyl azlactone in the presence of an acidic catalyst. Utilization of an isopropenyl azlactone rather than a vinyl azlactone eliminates the troublesome side reactions.
Although the above described methods for the preparation of acrylamide and methacrylamide functional polymers are quite satisfactory in many instances, the acidic catalysts utilized can sometimes have detrimental effects. For instance, certain polymers are prone to hydrolysis, degradation, or chain scission in the presence of acids, thus leading to a reduction in molecular weight with a corresponding loss of physical properties. Other polymers may have functional groups which can interact preferentially with the acid catalysts, thereby reducing or destroying their catalytic activity.
U.S. Ser. No. 316,234 also teaches that hydroxy functional oligomers and alkenyl azlactones may be reacted in the presence of strong bases such as tetrabutylammonium hydroxide and alkali metal hydroxides and alkoxides. However, these very strong, nucleophilic hydroxide and alkoxide bases can also lead to hydrolysis and polymer degradation reactions. Furthermore, they are capable of preferential reaction with the alkenyl azlactones themselves, resulting in undesired side reactions. U.S. Pat. No. 4,546,159 recommends the use of 4-dialkylaminopyridines as basic catalysts for the reaction of alcohols with alkenyl azlactones. Although these catalysts are more efficient than other basic catalysts previously described in the art, they still provide relatively slow reaction rates.
In assignee's copending patent application U.S. Ser. No. 910,528, filed Sept. 23, 1986, several tertiary amine catalysts are recommended as being effective for the reaction of polyols and saturated azlactones. This reference recommends 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) as the preferred catalysts.
U.S. Pat. No. 4,681,967 teaches a process for transesterification of a carboxylic or carbonic acid ester wherein the catalyst is either (a) a cyclic amidine (such a DBU or DBN), or (b) a Group V element-containing Lewis base and an epoxide. The Lewis base includes amidines, amines, and phosphines.